I recently used

$filename = <absolute path of the file>;
`echo "$a" > $filename`
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if double quotes surrounded by $a is missed, then if $a contains ;(semi colon) then the echo won't write into the file, the echo command just end with that semi colon, so double quotes around $a is must
for example the below won't write into a file

$a = "this is testing;";
`echo $a > $filename`
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it just creates a 0 byte file, because that semi colon just ends that echo command, and the redirection operator would have already created that file with 0 byte size
I spent some time on this issue so I am sharing this here
i am speaking in the context of *nix systems.